In the field of electrophotography, the heating roller method has been widely disseminated as a means for enabling a toner image formed on an image receiving sheet to be permanently fixed on the sheet.
This method is exceptionally fit for an electrophotographic copying device because the surface of a heating roller tightly contacts the image surface of the image receiving sheet and consequently the thermal efficiency with which the toner image is thermally fused to the image receiving sheet is so perfect as to permit quick fixation of the image.
In recent years, the electrophotographic copying devices of this class have come to demand a cut in power consumption and an addition to operating speed and have consequently created a need for a toner capable of being fixed at low temperatures.
In order that a given toner may be rendered fixable at low temperatures, the toner requires to lower the melting temperature thereof. It is, therefore, conceivable to use a resin of a low melting point such as, for example, a vinyl chloride resin as the binder resin component incorporated in the toner or to increase the content of a (meth)acrylic ester component in a styrene-(meth)acrylic resin, for example.
Indeed the use of this resin enables the melting point of the toner to be lowered. It nevertheless has the possibility of narrowing the temperature range in which the toner can be fixed without inducing either low-temperature or high-temperature offset (hereinafter referred to occasionally as "non-offset range") or failing to offer a fully satisfactory toner fixing ratio.
JP-A-61-117,564 discloses a so-called pulverized toner obtained by melting and kneading a binder resin with a coloring agent and other components and pulverizing the resultant blend and classifying the produced particles, which pulverized toner is characterized by containing as basic resins 90-30% by weight of an epoxy resin having a weight average molecular weight of not less than 2000 and 10-70% by weight of a styrene-acryl resin having a weight average molecular weight of not less than 50000 for the purpose of allowing the produced pulverized toner to enjoy fully satisfactory pulverizability, avoid emitting any offensive odor during the course of fixation, manifest perfect fixability, and produce only sparing fogging during the course of printing.
JP-A-59-129,862 discloses a flash fixing toner such that an image formed of this toner is fixed by a procedure of exposing this image to an ultraviolet light of high energy and a visible radiation thereby elevating the temperature of the toner in the image instantaneously to the melting point thereof, which flash fixing toner is obtained by composing a binder resin combining 100 parts by weight of an epoxy resin having a weight average molecular weight of 1000-10000 with 10-50 parts by weight of an ethylene-n-butyl acrylate resin having a weight average molecular weight of 10000-100000 for the sake of appropriate adhesiveness of the toner resin to the surface of a sensitive plate and for the purpose of preventing the flash fixed image from producing a void and then pulverizing the resultant binder resin.
Though it has been known in the art to use the epoxy resin as part of the binder resin in the toner of the kind under discussion, it has never been known to the art to use the epoxy resin for the purpose of enabling the toner to be fixed at low temperatures.
In the case of the so-called polymerized toner resorting to the suspension polymerization, the emulsion polymerization, or the like which is regarded as advantageous because of the uniformity and the fineness of the toner particles, the qualities yearned for in the light of the stability of charging of the toner and the high degree of resolution of the toner image, it has been heretofore considered difficult to incorporate the epoxy resin mentioned above in this polymerized toner by reason of the method of polymerization used for the epoxy resin.